Conventionally, a lamp device has been used which uses a GX53-type cap portion standardized by the IEC (International Electrotechnical Commission). The lamp device has a flat lamp device body, the GX53-type cap portion is provided on an upper face side of the lamp device body, a flat light source using a fluorescent lamp, LED or the like is arranged on a lower face side of the lamp device body, and a lighting circuit for lighting the light source is housed inside the lamp device body. On the cap portion, a pair of lamp pins each having a large diameter portion at its top end is projected. The lamp pins of the lamp device are inserted and hooked into a socket device by turning the lamp device, the lamp device is held by the socket device, and power is supplied from the socket device to the lamp pins (see, for example, Patent Document 1).